Inkjet printing is typically done by either drop-on-demand or continuous ink jet printing. In drop-on-demand inkjet printing ink drops are ejected onto a recording surface using a pressurization actuator (thermal or piezoelectric, for example). Selective activation of the actuator causes the formation and ejection of a flying ink drop that crosses the space between the printhead and the print media and strikes the print media. The formation of printed images is achieved by controlling the individual formation of ink drops, as is required to create the desired image. For continuous inkjet a pressurized ink source produces a continuous stream of ink drops and a deflection mechanism (electrostatic or air flow, for example) separates drops intended for printing from drops not intended for printing. The drops not intended for printing are caught in a gutter and either recycled or disposed.
Motion of the print medium relative to the printhead can consist of keeping the printhead stationary and advancing the print medium past the printhead while the drops are ejected. This architecture is appropriate if the nozzle array on the printhead can address the entire region of interest across the width of the print medium. Such printheads are sometimes called pagewidth printheads. A second type of printer architecture is the carriage printer, where the printhead nozzle array is somewhat smaller than the extent of the region of interest for printing on the print medium and the printhead is mounted on a carriage. In a carriage printer, the print medium is advanced a given distance along a print medium advance direction and then stopped. While the print medium is stopped, the printhead carriage is moved in a carriage scan direction that is substantially perpendicular to the print medium advance direction as the drops are ejected from the nozzles. After the carriage has printed a swath of the image while traversing the print medium, the print medium is advanced; the carriage direction of motion is reversed; and the image is formed swath by swath.
A drop generator in an inkjet printhead includes a chamber having an ink inlet for providing ink to the chamber, and a nozzle for jetting drops out of the chamber. Two side-by-side drop generators are shown in prior art FIG. 1 (as in US Patent Application Publication No. 2004/0263578) as an example of a conventional thermal inkjet drop on demand drop generator configuration. Partition walls 20 are formed on a base plate 10 and define chambers 22. A nozzle plate 30 is formed on the partition walls 20 and includes nozzles 32, each nozzle 32 being disposed over a corresponding chamber 22. Ink enters chambers 22 by first going through an opening in base plate 10, or around an edge of base plate 10, and then through ink paths 24, as indicated by the arrows in FIG. 1.
FIG. 2 is a schematic top view of the configuration of the prior art drop generator type shown in FIG. 1, but with additional labeling details not provided in US Patent Application Publication No. 2004/0263578. Chamber 22 includes an inlet 27 having a first edge 27a and a second edge 27b. Inlet 27 includes a width W between first edge 27a and second edge 27b. Chamber 22 includes a center 21 (indicated by the cross hairs). In this example, the nozzle 32 and the heater 13 have centers that are in line with the chamber center 21, but that is not always the case. First edge 27a and second edge 27b of inlet 27 are substantially symmetrically arranged relative to chamber center 21, i.e. they are disposed on opposite sides of the chamber center, relative to the inlet width direction. Ink path 24 (called a channel herein) includes an entry region 25, a neck region 26, and then a gradually wider region that connects to inlet 27 of chamber 22. Ink enters the chamber 22 from ink feed 35, an opening in the base plate 10 (called a substrate herein) through channel 24. When heater 13 is briefly pulsed, it vaporizes a portion of the ink, and the growing vapor bubble pushes a drop of ink out of nozzle 32. Some of the force of the growing bubble pushes ink backward through channel 24, but the chamber walls 29 and the increased fluid impedance due to neck region 26 are designed to direct a large portion of the force of the growing vapor bubble to eject the drop of ink. The vapor bubble is either vented through the nozzle 32, or it collapses in chamber 22. Capillary action and reduced pressure in the chamber draw ink in to refill chamber 22. The fluid impedance of channel 24 is designed to allow rapid refill for ejection of the next drop, as well as to direct vapor bubble forces toward drop ejection.
A drawback of prior art chamber configurations, including the configurations of FIGS. 1 and 2, is that some regions of the chamber, such as corners 28, can have relatively low fluid velocity during both drop ejection and ink refill. Air bubbles and particulates can accumulate in such low flow regions. Air can enter the chamber 22 through nozzle 32 or air can come out of solution from being dissolved in the ink. Unlike ink vapor bubbles which can condense completely, air bubbles are persistent unless they are removed from the chamber. Although very small air bubbles may not be a problem, as the air bubbles accumulate and grow, they can interfere with proper jetting. Similarly, small particulates that enter the chamber through channel 24 may not be a problem, but as they accumulate, they can also cause problems with proper jetting.
What is needed is a drop generator configuration and method of operation that does not allow air bubbles and particulates to accumulate in the chamber.